1. Field of the Invention
The present invention relates to a semiconductor device and a production process thereof. More particularly, the present invention relates to a technology for simplifying the production process and reduce production costs, while considering prevention of adverse environmental effects in the production of semiconductor devices in which a semiconductor chip such as a bare chip and the like is attached onto a wiring substrate.
2. Description of the Related Art
In the production of semiconductor devices, to ensure electrical functions of a semiconductor element such as an IC chip or an LSI chip (hereinafter referred to as a xe2x80x9csemiconductor chipxe2x80x9d) after its formation and dicing on a silicon wafer, it is necessary to electrically connect an electrode of the semiconductor chip to a wiring pattern formed on the wiring substrate. Further, as is well-known in the art, a wire bonding system and a flip chip bonding system have been suggested for carrying out such electrical connection (the so-called attachment of chips).
In the wire bonding system, after formation of a wiring pattern on a wiring substrate, a semiconductor chip is bonded to a wiring substrate by adhering a lower surface (surface opposed to a surface having an electrode) of the semiconductor chip through an adhesive to a surface (wiring-bearing surface) of the wiring substrate, followed by electrically connecting through a bonding wire the electrode of the semiconductor chip with the wiring of the wiring substrate.
On the other hand, in the flip chip bonding system, after formation of the wiring pattern on the wiring substrate as in the wire bonding system mentioned above, a metal bump such as a solder bump is applied to an electrode pad of the semiconductor chip, and the metal bump is electrically connected through an anisotropic conductive sheet or the like to the wiring (land) on the wiring substrate.
More specifically, in all of the prior art electrical connection systems, it was necessary to separately conduct the formation of the wiring and attachment of chips. Further, in the attachment of chips, it was necessary to use a bonding means such as solder and the like for the electrical connection between the chip and the wiring.
As described above, according to the prior art methods, since the fabrication step for forming a wiring on a wiring substrate and the chip attachment step for electrically connecting the wiring with a semiconductor chip have to be separately carried out in the production of semiconductor devices, there was a problem in that the production process for the semiconductor devices was relatively complicated and thus the production cost increased.
Further, in the case wherein the semiconductor devices are produced by using a wire bonding connecting system, since a bonding wire is applied at a higher position than the surface level of the electrode on the semiconductor chip, there was a disadvantage in that the total height of the semiconductor devices is relatively increased. Such an increase in the height of the devices goes against a recent trend of producing thin semiconductor devices.
Further, there was a similar disadvantage in the production of semiconductor devices in using a flip chip bonding system, in that the total height of the semiconductor devices is relatively increased, because a bump has to be applied between a wiring substrate and a semiconductor chip. This is because the total height of the semiconductor devices increases relatively in conformity with the size of the applied bump.
Furthermore, since a soldering process is frequently used for the bonding of a wiring circuit with a semiconductor chip in the attachment of the chip, there was a problem in that a solder used may cause environmental pollution.
The present invention is therefore directed to solve the prior art problems, described above, in the production of semiconductor devices.
The object of the present invention is to provide a semiconductor device which enables production of the device in a simplified process and at a reduced production cost, and also enables a reduction in the thickness of the device while inhibiting adverse effects on the environment, along with a production process for such a semiconductor device.
In one aspect thereof, the present invention resides in a semiconductor device comprising a wiring substrate having attached thereon at least one semiconductor element, in which the semiconductor element is directly bonded through a Coulomb""s force created between radicals of the atoms constituting the semiconductor element and reactive radicals on a surface of the wiring substrate, and a wiring pattern formed on the surface of the wiring substrate extends onto a surface of the semiconductor element and is electrically connecting with an electrode and the like formed on the surface of the semiconductor element.
In the semiconductor device according to the present invention, the semiconductor element used herein is not restricted to a specific one. As described above, the semiconductor element includes an IC chip, an LSI chip and others. Of course, if desired, the semiconductor device may include any other elements or parts which are conventional in the field of semiconductor devices. In the semiconductor device of the present invention, it is preferred that the wiring pattern, when viewed at an end portion of the semiconductor element, have a planarized surface. In other words, the height of the wiring pattern at such an end portion should be substantially the same as that of the wiring pattern on the wiring substrate, and the wiring pattern extends on a substantially even surface, i.e. there is no substantial step in a surface of the wiring pattern at the end portion of the semiconductor element adjacent to a portion having no underlying semiconductor element. Since the wiring pattern can be produced to have a substantially flat surface and a reduced thickness, it becomes possible to produce thinner semiconductor devices along with prevention of permeation of a liquid into a gap between the wiring pattern and the underlying semiconductor element or wiring substrate.
Further, the semiconductor element may be used without applying any surface treatment to a bonding surface thereof, but it is preferred that the bonding surface of the semiconductor element be subjected to a mirror polishing process to make a mirror surface. Since a Coulomb""s force can be increased in proportion to the surface area of the bonding or contacting surface of the semiconductor element, increased bonding strength of the element can be obtained when the contacting surface is increased as a result of the polishing process.
A surface of the wiring substrate to which a semiconductor element is bonded is preferably activated with a plasma treatment to generate reactive radicals on the surface thereof. In this plasma treatment, if the wiring substrate used is made of a silicon substrate, it is preferred that the plasma treatment be carried out with irradiation of ultraviolet radiation.
Further, to ensure good electrical communication between an upper surface and a lower surface of the wiring substrate, it is preferred that the wiring substrate further have a conductor formed through the substrate. In the conductor formed through the wiring substrate, it is preferred that an end surface of the conductor exposed at a semiconductor element side of the wiring substrate be electrically connected with a wiring pattern formed on the wiring substrate. On the other hand, it is preferred that another end surface of the conductor exposed at another surface of the wiring substrate opposed to the semiconductor element side further comprise an external connecting terminal, although the other end surface of the conductor may be directly used as an external connecting terminal without applying any conducting means.
Furthermore, it is preferred that the wiring substrate and thus the semiconductor device have a protective insulating layer as the uppermost layer. The presence of the protective insulating layer is effective in protecting the semiconductor element, the wiring pattern and the like from undesirable external influences such as permeation of water and to prevent electrical defects such as disconnection and short-circuiting.
The semiconductor device of the present invention may further comprise one or more additional semiconductor elements in addition to the semiconductor element directly attached to the wiring substrate. The additional semiconductor element can include any elements or parts conventionally used in semiconductor devices. For example, the additional semiconductor element can be attached to the wiring substrate, after an opening portion is formed at a predetermined site of the insulating layer covering the semiconductor device to expose the underlying wiring pattern. The additional semiconductor element is attached through the opening portion to the semiconductor substrate. The opening portion may be filled with any electrical conducting material to assist in bonding between the semiconductor element and the wiring pattern.
In the semiconductor device of the present invention, the wiring substrate and the semiconductor element both may be formed using any substrate materials, depending on the details of the semiconductor device to be produced and other factors. However, to attain good electrostatic adhesion based on the Coulomb""s force, it is preferred that the wiring substrate and a substrate of the semiconductor element have substantially the same crystal orientation direction. For example, it in preferred that the wiring substrate be made using a glass substrate, and the semiconductor element be made using a silicon substrate. It is also preferred that the wiring substrate be made using a silicon substrate having an insulating layer, generally a silicon oxide layer, on a surface thereof, with the semiconductor element being made using a silicon substrate. Of course, other substrate materials may be used, if desired.
The semiconductor device of the present invention may be used alone or in combination of two or more devices. For example, two or more semiconductor devices may be stacked to form a semiconductor device having a three-dimensionally extending attaching structure. In this attaching structure, it is preferred that an external connecting terminal and the like formed on the wiring substrate of an upper semiconductor device be electrically connected with the wiring pattern of a lower semiconductor device adjacent to the upper semiconductor device.
In another aspect thereof, the present invention resides in a process for the production of a semiconductor device comprising a wiring substrate having attached thereto at least one semiconductor element, which process comprises the steps of;
generating reactive radicals on a surface of the wiring substrate;
directly bonding the semiconductor element, through a Coulomb""s force created between radicals of the atoms constituting the semiconductor element and the reactive radicals, on a surface of the wiring substrate; and
forming a wiring pattern at a predetermined pattern on the surface of the wiring substrate, the wiring pattern extending onto a surface of the semiconductor element and electrically connected with an electrode and the like formed on the surface of the semiconductor element.
As will be easily appreciated from the above description of the semiconductor device, the production process for the semiconductor device according to the present invention can be advantageously carried out in different embodiments.
For example, a wiring pattern is preferably formed in such a manner that the wiring pattern covers an end portion of the semiconductor element, while maintaining a planarized and thus substantially flat surface over the semiconductor element and the adjacent wiring substrate. Preferably, the flat surface is obtained in the wiring pattern by depositing the wiring pattern at a relatively high thickness, in accordance with a so-called xe2x80x9cthick-layer forming methodxe2x80x9d. The thick-layer forming method is preferably carried out by depositing, in sequence, at least two plating layers on the wiring substrate. More preferably, the wiring pattern is formed as a two-layered plating layer by using a combination of an electroless plating step and the subsequent electrolytic plating step. For example, the wiring pattern can be formed in a relatively high-thickness by plating a Ni layer, followed by plating a Cu layer.
In particular, the production process for the semiconductor device of the present invention can be advantageously carried out by the steps of:
forming a conductor, to a lower end surface of which an external connecting terminal is electrically connected, in a region surrounding a chip mounting area on a surface of a glass substrate;
treating, in a vacuum, a surface of the portion, corresponding to the chip mounting area, of the glass substrate with plasma to form an activated surface in the glass substrate;
bonding, in a vacuum, a silicon chip through its surface, opposed to an electrode-bearing surface of the same, to the activated surface of the glass substrate;
forming a wiring pattern having a predetermined configuration on the glass substrate in such a manner that an upper end surface of the conductor in the glass substrate is electrically connected with the electrode of the silicon chip; and
polishing a lower surface of the glass substrate opposed to a pattern-bearing surface thereof until a lower and surface of the conductor is exposed from the lower surface, thereby providing a thinned glass substrate having a predetermined thickness.
Further, it is preferred that the production process further comprise the step of bonding a metal bump as an external connecting terminal to the lower end surface of the conductor after thinning of the glass substrate.
Furthermore, it is preferred in the production process that, prior to bonding the silicon chip to the activated surface of the glass substrate, a surface of the silicon chip opposed to an electrode-bearing surface thereof be plasma treated to provide the activated surface.
According to the semiconductor device and its production process according to the present invention, a silicon chip in the form of a thin film can be directly bonded to a glass substrate by utilizing an adhesion phenomenon (electrostatic adhesion) based on a Coulomb""s force between reactive radicals on the glass substrate and silicon (Si) atoms of the silicon chip, since a surface of the glass substrate is treated with plasma to activate the surface, thereby producing reactive radicals around the activated surface, while the silicon chip is constituted from the Si atoms. At the same time, in the formation of a wiring pattern to be electrically. connected through a conductor or electrically conducting means to an external connecting terminal of the glass substrate, a connection between the wiring pattern and an electrode of the silicon chip, i.e. attachment of chip, can be carried out simultaneously. More specifically, according to the present invention, the formation of the wiring pattern and the attachment of the chips, which were both separately carried out in the case of prior art devices, can be carried out in a single step for forming a wiring pattern.
Thus, according to the present invention, it becomes possible to simplify a production process and to reduce the production cost. Moreover, because specially produced bonding or adhering materials such as solder or anisotropic conductive sheets, which were essential to the prior art devices, are not required in the practice of attachment of chips, the production cost can be further reduced.
Further, since a thin silicon chip can be directly adhered (attached) to the glass substrate without using a bonding wire used in the wire bonding connection system or a bump used in the flip chip connection system, it becomes possible to reduce the total thickness of the resulting semiconductor device.
Furthermore, since a Coulomb""s force-based adhesion (electrostatic adhesion) process and a conventional wiring pattern formation process are used in the adhesion step in the chip attachment and the subsequent connection step of the chip with the wiring pattern, i.e. since a xe2x80x9csolder which can adversely affect environmental conditionsxe2x80x9d, which was conventionally used in the prior art, is not used in the practice of the present invention, environmental pollution can be effectively prevented.